1. Technical Field of the Invention
The present invention relates to speed limiting for a water turbine or other water motor driven sprinklers, and more particularly, to a method and apparatus which employs dynamic viscous braking to control the water motor output shaft speed in an improved, convenient and reliable manner. By employment of the invention, a sprinkler system can be winterized by purging water from the system using high pressure air, without the risk of damage to the rotating parts due to overspeeding, while also preventing overspeeding due to other causes such as a clogged pressure relief bypass mechanism. In addition, the invention can be employed to limit the maximum rotational speed of the water motor, thereby simplifiying the design and construction of the transmission used to couple the water motor to the rotating nozzle.
For simplicity, the invention will be described in the context of a gear driven sprinkler powered by a water driven turbine, but it is to be understood that the invention is also applicable to and comprehends within its scope, reversing sprinklers and/or sprinklers having other types of water driven motors to rotate a distribution nozzle.
2. Relevant Art
As explained in above-referenced U.S. patent application Ser. No. 10/141,261, sprinkler systems in northern climates must be drained or blown-out with air to prevent damage due to freezing. In systems powered by water driven turbines or the like, excessively high turbine shaft velocities can result when run with compressed air, because it is both relatively light compared to water, and expands across the turbine stator onto the turbine blades, in contrast with water, which is incompressible. Typical turbine driven sprinklers run 10-15 times faster when powered by compressed air (30-50 psi) than in normal operation with water.
High turbine shaft velocities can heat the shaft and cause it to seize to the plastic housing material. This prevents the turbine from turning and renders it unusable in the future unless care is taken to limit the system air, blow-out time and pressures. This has proved to be one of the major causes for premature failure of gear driven sprinkler in colder climates, where sprinklers are used for only part of the year, and should last much longer than in warmer climates where they are run year round.
Devices are known for controlling the rotational speed of turbine-driven sprinklers. One such device, shown in Clark U.S. Pat. No. 5,375,768, is designed to maintain constant turbine speed despite variations of inlet water pressure. The patented sprinkler relies on a throttling device to direct part of the water to the turbine rotor, and a pressure responsive valve to divert some of the water around the turbine. This design, however, and other known designs can not effectively limit rotational speed when the turbine is driven by a compressible fluid such as air, and still allow the turbine to run at a sufficiently high speed when it is driven by an incompressible fluid such as water because of the rapid expansion of the compressed air as it enters the turbine chamber, so far as applicant is aware.
The invention disclosed in above-referenced U.S. patent application Ser. No. 10/141,261 addresses the problems of overspeeding during winterization by providing a construction in which the turbine flow discharge area is substantially the same as or only slightly larger than the inlet stator area. In an alternative construction, the inlet stator flow area is separated from the turbine blades by a flow bleed area to bleed off a significant portion of the expanding air flow, i.e., the flow normally directed onto the turbine during water driven operation and in standard configuration sprinklers, before a portion of the air is deflected to strike the turbine blades to produce the turbine rotation.
In the above-described designs, water, being incompressible, does not experience expansion after flow through the stator inlet flow area and does not flow out the intermediate bleed but continues in its line of flow to be directed onto the turbine blades to run the turbine in a normal manner. In the case of air (compressible flow) the portion remaining after the intermediate bleed can be limited to just enough to turn the turbine at its normal speed as when water-driven.
Another known speed related issue in sprinkler systems concerns bypass mechanisms such as shown in the above-mentioned Clarke patent which are intended to maintain constant turbine shaft speed independent of inlet water pressure variations. If these devices become obstructed or otherwise malfunction, undesirable speed variations, and in extreme cases, damage to the rotating parts can occur.
Yet a further known speed related issue involves design of the power train which couples the turbine to the sprinkler head. Sprinkler systems often include sprinklers having nozzles with different flow rates and water travel distances to accommodate the shape and size of the area being irrigated. The turbines must therefore be designed for efficient momentum transfer from the flowing water to assure adequate torque for driving the sprinkler head under all operating conditions. For this reason, the turbines typically rotate at speeds in the range of about 1,000 to 2,000 RPM or higher. The rotational speed of the nozzle, however, is typically in the range of about 1-2 RPM, for short or medium range sprinklers, or even less for long range sprinklers, as the circumferential speed of the water stream limits the effective range. To achieve such low speeds, larger gear reduction is needed than required to past provide the necessary driving torque for rotating nozzle drive shaft which may be ⅜ to 1 inch diameter to conduct the water to the propagation nozzle which is being rotated. If the turbine could conveniently be made to run slower, the gear reduction mechanism could be simplified less part and further reduced in size. This could be an important incentive for promoting more widespread use of gear driven sprinklers which provide more uniform coverage and lower water use than the spray heads now used in a majority of the irrigations system sprinklers.
A need clearly exists for an approach to speed control which addresses all of these problems in an integrated manner.